Material Lift Processes (MLO) are commonly used in semiconductor manufacturing to remove undesired material. This step is employed when wet chemistries, such as aggressive etchants, would attack the substrate itself or attack other exposed materials in the film stack that are desired to be left unchanged. The sequence to perform the MLO is to apply Photo resist to a substrate (circuit board or wafer) and then expose with light through a mask. The photo resist is then developed via exposure to a chemistry that will remove a portion of the resist. The result is a substrate partially protected by photo resist and partially exposed. The substrate then sees material deposition (metal, dielectric . . . ) across the entire wafer surface. This added film adheres in locations to the substrate and in other locations to the photo resist. At this point the MLO step occurs. Solvents are typically employed to swell and then remove the resist from the substrate. During this step, the deposited film (metal, etc.) that was deposited on the photo resist also is removed when the resist is removed. The deposited film that adhered to the substrate remains in place. The solvent is typically collected for re-use. Post MLO, the substrate is rinsed and then dried. The substrate is ready for the next step in the manufacturing process.
The metal (material) that was removed (or lifted off) is rinsed from the wafer surface with pressurized solvent. This metal is left to follow the drain path of the recycled solvent. In cases this metal is in large pieces and will be caught by large mesh strainers within the process chamber. Those pieces of metal smaller than a chamber drain screen mesh (or all materials if no drain screen is in the tool design) pass through the drain into the recycle chemistry loop. These materials will cause damage to hardware (such as pumps), clog filters and will cause yield loss on subsequent wafers processed (such as pitting or scratching) if not removed from the recycle solvent stream.
Historically strainers have removed the bulk of this lifted material from the solvent stream. The size and quantity of materials is dependent of the wafer design. In high volume manufacturing it is not uncommon for strainers to require maintenance several times per week. Properly designed and sized re-usable strainers will capture the majority of material and permit expensive, single use, low pore size filters to have lives of one or more months.
Maintenance to clean separated material from the strainers has historically required the tool to be shut down. Maintenance personnel have been required to remove, empty and clean the solvent wetted strainers on a routine basis. The industry has requested a no down time strainer design that is more efficient at removing materials that can cover a variety of pore sizes with a single design that requires less maintenance and that does not expose personnel to solvents.